Just try it. Close your browser, disconnect your computer from internet, open the browser and try to access your data on LastPass. It will be there. Because it's stored locally.

In this forum topic, Joe Siegrist says that LastPass always store the most updated local copy of our encrypted data. I wonder if that data is stored at a single file and where. So I could make backup copies of it and maybe even access it through LastPass Pocket.

I actually asked this same question in there, twice now, but still got no answer!

Linux

Firefox on other platforms utilizes
the Firefox profile directory. You can
back these directories up and they
will contain the latest copy of your
data. Be aware that on Windows,
Protected Storage is used to
additionally encrypt your data so
you'll need to ensure that your
Windows profile is also backed up.
Your data is also backed up at
LastPass.com, and a copy is on any
other device or computer you use so
this isn't required.

Your data is also securely synced with
the LastPass servers. When you make
updates to your LastPass account,
whether in a browser plugin, a mobile
app, or the website, the updates are
reflected on the LastPass servers as
well. If your hard drive ever crashes,
or your laptop is stolen, you can
easily regain access to your accounts
by reinstalling the plugin
(https://lastpass.com/download.php)
and logging in with the same email and
master password.

we can easily "regain access" simply by logging in on the website! I had found those folders before, and it's good to now be sure the data I'm looking for must there somewhere. But I still don't know which files specifically are related to the question.
–
CawasMay 9 '11 at 14:29

This is the main quote I was missing: "You can back these directories up and they will contain the latest copy of your data"
–
CawasMay 9 '11 at 20:21